support/download: fix the git helper

Re-add the git_done variable (lost in commit [1]).

Fixes download problem reported by Rohit Kumar [2].

[1] http://git.buildroot.net/buildroot/commit/?id=7e40a1103a919a8177f00ddca2b46b4439953511
[2] http://lists.busybox.net/pipermail/buildroot/2014-August/103733.html



Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>